Telecommunications requirements have evolved tremendously since multiplexed T1 was first installed in the early 1960s. T1 was originally developed to carry 24 digitized voice channels in a time division multiplexed (TDM) format. With the evolution of computers and the resulting data communication requirements, Digital Data Service (DDS) circuits were offered directly to customers beginning in the early 1970s. One or more DDS circuit would reside in one of the 24 channels within a T1.
Conventionally a T1 circuit carries subrate DDS traffic at rates as low as 1200 bits/second. Carrying subrate traffic is accomplished by first multiplexing the subrate traffic with or without other subrate channels to the DS0 rate of 56 kbit/second. These DS0s are then multiplexed to the T1 rate.
With the proliferation of computers and the corresponding data communications requirements since the 1980s, multiple channels within a T1 (referred to as N-by-56 kbit/second service) and even entire T1s are now offered directly to customers. As a result, the ability to send technicians into the field to test every problem has become a substantial problem.
Carriers have begun installing automated cross-connect switches in order to allow quick re-routing of circuits as demand increases or failures occur. A side benefit of these switches is that they offer T1 test ports which provide up to 24 channels which can be tested at one time. Typically, more than 500 T1s, each containing up to 24 channels are switched by these cross-connects. Any of these channels can be rounted to one of typically 10 to 30 T1 test ports. In order to test channels rounted to these T1 test ports, the T1 signal at the T1 test ports must be demultiplexed into individual channels and then test equipment must be provided for each channel. This results in an enormous requirement for test equipment, as well as for power and floor space since test equipment for each channel must be duplicated for each test which is to run simultaneously.
DS3 and higher rate multiplexing schemes are currently used operating at data rates of 44 Mbit/s and higher. These multiplexing schemes are all based on the T1 multiplexing level. A DS3 circuit conventually carries DS0 traffic. These DS0 s would have been multiplexed to T1 then DS2 and finally to the DS3 rate.
In Europe the CCITT hierarchy uses multiplexing rates based on a 2048 kbit/s standard. Again, these 2048 kbit/s signals are multiplexed to data rates of 34 Mbit/s and higher.
Voice frequency (VF) encoding techniques have improved to the point that a standard eight bit channel within a T1 signal capable of actually support two VF communications paths. The equipment is being installed to carry VF using only four bits divides the T1 TDM format into what is in effect 48 channels. This results in twice the traditional number of voice connections being carried in the same T1 bandwidth.
With the increased need for data communications, conventional multiplexers are being installed not only in the network, but within the customer premises as well. As a result, there is a large demand for testing multiplexers by manufacturer and installers. Conventionally this would be done by "multiplexing-up" test data transmitted into one or more low speed channels to the higher TDM rate. The TDM output would then analyze one channel at a time by dropping the channel out of the multiplexed TDM data path. These test are time consuming when the number of channels to be tested is large.
In some applications the bandwidth available in a communications link is so limited that digital speech interpolation (DSI) is used so that bandwidth available during pauses in speech bursts can be allocated to another speaker. The time division allocation of this technique changes so rapidly that testing is typically not performed on the link between the DSI units.
The information discussed above is hereby incorporated by reference.